Single phase induction motors have been widely used due to their construction simplicity, rigidity and high performance. They are particularly useful for low power range applications such as household appliances, refrigerator hermetic compressors, washing machine motors, fans. They are also used in some industrial applications as well.
Single phase induction motors generally have a cage type rotor and a coiled stator, basically having two windings, one being for the main coil and the other for the starting coil. When an alternating voltage source is applied only to the main coil of a single phase induction motor, a magnetic field fixed in space and alternating in magnitude is created which does not develop any torque to start the rotor movement. Accordingly, it is necessary to create a turning magnetic field to take the rotor from the static condition so that the starting is initiated.
Such turning magnetic field can be achieved by feeding the starting coil with a current time-displaced to the current flowing in the main coil, preferably at an angle as near as possible to 90.degree.. The current phase angle in the starting coil in relation to the current in the main coil can be achieved by construction differences such as by designing the starting coil with a power factor higher than that of the main coil. It also can be achieved by installing an external impedance connected in series to the starting coil, the impedance usually being a resistor or a capacitor. It is necessary to install a device in series with the starting coil circuit which temporarily connects this circuit to the power source until the rotor has reached a speed near to the synchronous speed, thereby giving rise to the starting.
In motors in which a much higher efficiency is required, the starting coil is not fully disconnected from the voltage source after the starting, with a capacitor being kept in series with this winding. The running capacitor, or permanent capacitor, allows a current flow much lower than the current during the starting, but keeps a double phase feed condition for the motor, thereby increasing its maximum torque and increasing its efficiency.
For a motor having such configuration, employing a permanent impedance, there is known a type of starting device as described in the Brazilian patent document PI 201.210 consisting of a resistor having a positive temperature coefficient (PTC). This PTC resistor device, which is connected in series to the starting coil, has a low resistance at room temperature, thereby allowing a high current to flow during the starting. After a predetermined time, due to a self heating effect, it has a very high resistance, practically acting as an open circuit. During normal motor running, the voltage drops across the PTC is relatively high, and the resistor is kept hot, thereby dissipating a amount of power varying from 1.5 to 5 W or higher, depending upon the form of the resistor and motor and the room temperature. Such power expended by the PTC during the entire motor running period decreases the motor efficiency, thus being undesirable.
Another known starting device is a current relay which keeps the starting coil circuit on while the current in the main coil is high. Although it is a very simple device and does not consume any power during the normal motor running period, it is not advantageous because it cannot be used in configurations having a permanent capacitor. Such a fact is due to the delays inherent to relays which cause the contacts thereof to close right after the motor has been powered. At this time, the permanent capacitor will already have stored a high amount of energy and such energy will be dissipated when contacting the relay at the time it is closed thereby quickly causing its destruction.
Another known starting device is described in U.S. Pat. No. 4,605,888 consisting of an electronic circuit which uses a TRIAC. Although consuming a small quantity of power during the normal motor running, it cannot readily be used jointly with a permanent capacitor.